Cut what the audit found: dead columns, one-time upgrades, three deps

Works through TODO.md from the 2026-09-12 over-engineering audit. Drops the
entries.read/flagged/position columns (migrate() removes them from older
databases), migrate_opml_children, the legacy interval_mins key, the
contrib/ systemd units, test-only Db wrappers, a duplicate token generator,
redundant logbuf visitors, unused page state and CSS, and the infer, dirs
and tokio-stream dependencies. The icon is served once as /icon.png instead
of inlined four times, taking about 94 KB off the two pages.

The adoption's subscription half was not dead: it gives a fresh install's
first admin the config's feeds. It stays as adopt_catalogue, now tested.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TAC7sLVqfKmY6rsTLXzNgk

docs/architecture.md

@@ -58,9 +58,9 @@ entry_state    user_id, feed_id, guid, read, flagged, position
                                                          PK (user_id, feed_id, guid)
 ```
 
-`entries` still has `read`, `flagged` and `position` columns from before accounts existed. They are
-**dead** — the migration copied them into `entry_state` and nothing reads them now. Anything found
-querying them is a bug; two were.
+Read state is `entry_state` alone. `entries` had `read`, `flagged` and `position` columns from
+before accounts; two bugs came from queries still reading them, and `migrate()` drops them from an
+older database.
 
 Schema changes: add the table or column to `SCHEMA`, and for a column also to the list in
 `migrate()`, which does `PRAGMA table_info` then `ALTER TABLE ADD COLUMN`. `Db::memory()` runs the